We report Hubble Space Telescope observations of dramatic variability in the reflection nebulosity of HH 30, a compact bipolar nebula which is a nearly edge-on accretion disk system. A lateral asymmetry, absent in 1994 images and only weakly seen in 1995, has grown to the extent that one side of the northeastern reflection nebula appeared 4 times brighter than the other in R band images made in spring 1998. The timescale of the variability is much shorter than disk dynamical timescales at the projected radius where the asymmetry is observed, and argues for an origin via variable illumination patterns projected onto the outer disk by changes in the inner disk or at the central star. Orbital motion of coherent clumps or voids in the inner disk near 1 AU might produce such an effect. Recently proposed models with magnetic accretion onto a young star through auroral zones misaligned from the stellar rotation axis can also produce broad beams of light sweeping across the disk. Simulated images of a disk illuminated by such a central ``lighthouse'' are a reasonable match the bright lateral asymmetry in the upper nebula of HH 30. However, the lack of a point-symmetric brightening in the southwestern counter-nebula would require significant differences in the accretion rings between the two magnetic poles. Intensive temporal monitoring of the system is needed to better characterize the variability and establish its physical origin.
American Astronomical Society Meeting Abstracts
- Pub Date:
- December 1998